Genomes and subgenomic clones of herpesviruses, human cytomegalovirus (HCMV and human herpesvirus-6 (HHV-6), were studied for oncogenic potential in order to evaluate their role in human cancers. CMV: Multiple regions of CMV strain Towne were studied for transforming functions by introducing DNA into rodent cells and human cells, and assayin the selected cells for tumorigenicity by growth in agarose or in nude mice. In this study, the major immediate early gene (IE1), which is a candidate subunit vaccine was also included. We identified two domains, mtrII and mtrIII, in the CMV genome that could transform cells to tumorigenicity. Th mtrII transformed human cells could produce poorly differentiated carcinoma in mice. Both domains contain several protein coding sequences. Data from the digestion of these domains with restriction enzymes suggested that a 79 amino acid (aa) protein in mtrII and a 270 aa protein in mtrIII might be responsible for tumorigenicity. Further mapping is in progress. We found that the IE1 gene is not involved in transformation. HHV-6: Similar studies were applied to HHV-6 to elucidate its role in huma lymphomas especially in AIDS and bone marrow transplant patients. Genomic DNA of HHV-6 isolates from AIDS patients (GS and Z29) and from chronic fatigue syndrome patients (GD) could transform mouse fibroblasts. Two DNA clones (21 kb and 8.7 kb) of GS isolate could transform human keratinocytes HHV-6 DNA, specific to 8.7 kb region, has been detected in an immunoblastic lymphoma by hybridization. Further studies are in progress to determine th specific genes involved in transformation. Identification and characterization of herpesvirus oncogenes are important to safety evaluatio and to development of CMV and other herpesvirus vaccines as well as to development of herpesvirus-based vectors for gene therapy. Since IE1 of CM is found non-tumorigenic in our assay, it may be safely used as a candidate subunit vaccine. If a tumor antigen is identified, a specific diagnostic reagent can be developed for CMV or HHV-6 related malignant diseases.